Scratch Programming - Rutgers CS

1

Scratch Programming

Lesson 5: Multiple Choice Decisions

Jt Scratch Lesson 5 Fall 2011 • slide 2

Problem

! Write a script to display a letter grade based

on a numeric score a student received in a

class where the typical scale is used.


Problem




! For example: a 97 gets an A; a 72 gets a C


Problem




! For example: a 97 gets an A; a 72 gets a C

! Lets try out a full multimedia version of the

code: Demo: Grading


Essence of the Problem

! The script must decide what letter grade to

assign




assign

2




assign

! Or put another way, it must decide whether

or not to give an A; decide whether or not to

give a B and so on




assign



give a B and so on

! What do you use in Scratch to decide

things?




assign



give a B and so on

! What do you use in Scratch to decide

things? An IF or IF/ELSE



! The script must decide what letter grade toassign

! Or put another way, it must decide whetheror not to give an A; decide whether or not togive a B and so on

! Use an IF or IF/ELSE

! By the way, we essentially have to make amultiple choice decision. We will comeback to this.


The Simple Stuff

! Variable: Score


The Simple Stuff

! Variable: Score

person’s numeric

score

3


The Simple Stuff

! Variable: Score

! Start & Input:


The Simple Stuff

! Variable: Score

! Start & Input: Ask for Score


The Simple Stuff

! Variable: Score


! Output:


The Simple Stuff

! Variable: Score


! Output: Say Letter


The Simple Stuff

! Variable: Score




The Simple Stuff

! Variable: Score



and so on

4


The Simple Stuff

! Variable: Score



and so on

appropriate letter is said


The Simple Stuff

! Variable: Score




The Decisions

! Decide if they get an A:


The Decisions



The Decisions



The Decisions

! Decide if they get an A: appropriate condition?

5


The Decisions



The Decisions

! What are the rest of the decisions like?


The Decisions

! What are the rest of the decisions like?

Pretty much the same


The Decisions

! Decide if they get an B:


The Decisions



The Decisions


6


The Decisions

! Decide if they get an C:


The Decisions



The Decisions



The Decisions

! Only two letters left.


The Decisions

! Only two letters left. What structure can we

use to decide between two options?


The Decisions



7


The Decisions




The Decisions



The Decisions

! Only two letters left. Condition?


The Decisions



The Decisions



The Complete

Script

8


The Complete

ScriptA?

B?

C?

D?

or F?

Four separate IFs


The Complete

ScriptA?

B?

C?

D?

or F?

Four separate IFs

Demo: Grading 1


Fixing the Script

! What is wrong?


Fixing the Script

! What is wrong?

! Scripts always run all the way through.


Fixing the Script

! What is wrong?

! Scripts always run all the way through. Just

because the first IF figured out an A should

be said, doesn’t mean the code stops.


Fixing the Script

! What is wrong?

! Scripts always run all the way through. Just

because the first IF figured out an A should

be said, doesn’t mean the code stops. The

code still goes on to the 2nd IF and then the

3rd and so on ….

9


Fixing the Script

! So lets consider the 2nd decision carefully:


Fixing the Script



Fixing the Script


See anything wrong here?


Fixing the Script


Works for 84.


Fixing the Script


Works for 84. Works for 86.


Fixing the Script


What about 92?

10


Fixing the Script


What about 92? Whoops!


Fixing the Script

! We have not been precise enough.


Fixing the Script

! We have not been precise enough.

The condition is not exactly right.


Fixing the Script

! What must be true about the score?


Fixing the Script

! What must be true about the score? Yes it must

be more than 80 ….


Fixing the Script

! What must be true about the score? Yes it must

be more than 80 …. but also less than 90

11


Fixing the Script

! Yes it must be more than 80 …. but also less

than 90. Both of these conditions must be true.


Fixing the Script



What do you use to combine two conditions

together …


Fixing the Script



What do you use to combine two conditions

together … and both must be true?Jt Scratch Lesson 5 Fall 2011 • slide 64

Fixing the Script



Use an AND


Fixing the Script



Use an AND


Fixing the Script



Use an AND

12


Fixing the Script

! Lets adjust the other two conditions also


Fixing the Script


a C is above 70 but also less than 80


Fixing the Script


a D is above 60 but also less than 70


Revised Script


Revised ScriptA?

B?

C?

D?

or F?


Revised Script

Demo:

Grading 2

A?

B?

C?

D?

or F?

13


Fixing the Script Again

! Close but not quite.



! Close but not quite. Now what is wrong?




Remember, just because an earlier IF is true

and causes a grade to be said - this doesn’t

mean the script stops.






mean the script stops. It eventually makes it to

the 4th decision (IF) where the F seems to be

said too.






mean the script stops. It eventually makes it to

the 4th decision (IF) where the F seems to be

said too. Why?



! Even after the A is said, it still eventually

makes it to the 4th decision (IF) where the F

seems to be said too. Let’s look at this 4th IF.

14



! Even after the A is said, it still eventually

makes it to the 4th decision (IF) where the F

seems to be said too. Let’s look at this 4th IF.



! Its an IF/ELSE.



! Its an IF/ELSE. Is it ever possible for an

IF/ELSE to do nothing?




IF/ELSE to do nothing? NO!




IF/ELSE to do nothing? NO! It always does

one of its two parts.



! Is it ever possible for an IF/ELSE to do

nothing? NO! It always does one of its two

parts. So either a D or F will always print !

15



! So either a D or F will always print ! Every time

we run this script, no matter what else it does - a

D or F will always be the last item to print.



! How can we fix this?



! How can we fix this? Lets not use and IF/ELSE.



! How can we fix this? Lets not use and IF/ELSE.

We can break this into two separate regular IFs

(like we did for the earlier grades.)



! Two separate regular IFs.



! Two separate regular IFs. No ELSE used.

16


Latest Script


Latest Script

Five Separate IFs

1. A?

2. B?

3. C?

4. D?

5. F?


Latest Script

Five Separate Ifs

Three of them had to

use ANDs

1. A?

2. B?

3. C?

4. D?

5. F?


Latest Script

Five Separate Ifs

Three of them had to

use ANDs

Bit more complicated

than our first

attempt.

1. A?

2. B?

3. C?

4. D?

5. F?


Latest Script

Demo:

Grading 3

1. A?

2. B?

3. C?

4. D?

5. F?


Latest Script

Demo:

Grading 3

It finally works right.

1. A?

2. B?

3. C?

4. D?

5. F?

17


Reviewing The Issue

! There was a fairly simple problem. Assign a

letter grade given a numeric score.

! Our original code seemed simple enough. But

the simple approach just didn’t work well.

! In the end, to get it to work right, we needed

five separate IF statements, many of which

needed to use AND conditions.


Reviewing The Issue

! There was a fairly simple problem. Assign a

letter grade given a numeric score.

! Our original code seemed simple enough. But

the simple approach just didn’t work well.

! In the end, to get it to work right, we needed

five separate IF statements, many of which

needed to use AND conditions.

! Why?


Insight

! The main issue is each IF is a separate statement

- each making its own decision. So, that means

more than one of them can come out TRUE.


Insight




! But, we don’t want more than one to be TRUE.


Insight





! We have a five way decision. We want exactly

one of five possible grades to be assigned.


Insight





! We have a five way decision. We want exactly

one of five possible grades to be assigned.

! Yet, IF/ELSEs by their nature, can only make a

two way decision.

18


Another Approach

! Lets look at the skeleton of an IF/ELSE that

makes a two-way decision but assigns the right

grade.


Another Approach



grade.


Another Approach



grade.

A?

or not?


Another Approach



grade.

A?

or not?Indicate person gets an A


Another Approach



grade.

A?

or not?Indicate person gets an A

Indicate which of the other

grades the person gets


Another Approach



grade.

Indicate person gets an A



19


Another Approach



grade.

So, how do

this?





Side Lesson

! What statements can you place inside the

sections of an IF/ELSE statement?


Side Lesson

! What statements can you place inside the

sections of an IF/ELSE statement?

Answer:

any type

of statement

can placed

here


Side Lesson

! But an IF statement is a type of statement.

Answer:

any type

of statement

can placed

here


Side Lesson

! But an IF statement is a type of statement. So an

IF can be placed inside another IF.

Answer:

any type

of statement

can placed

here


Example: Problem

! Tuesday is a special

day at a movie

theater. Everyone

gets in for $6, gets a

free drink of their

choice, and kids

under 10 years old,

get a free gift.

20


Example : Code Segment


day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift.

Give Toy To Kids


Example : Focus on Toy

! Not everyone gets the

toy.




toy. You only give it

to some people.





to some people. You

have to decide to give

it or not.





to some people. You


it or not. What do

you use in Scratch to

make a decision?





to some people. You


it or not. What do

you use in Scratch to

make a decision? Use

an IF.

21



! Use an IF. What is

our condition? What

has to be true about

the person for them

to get a toy?



! Use an IF. What is

our condition? What

has to be true about

the person for them

to get a toy?




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift.

Give Toy To Kids

Back to our main theater code




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift.

Give Toy To Kids

We have to use the code we

just created for the toy here




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift. We have to use the code we

just created for the toy here




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift. Here we have an IF

22




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift. Here we have an IF

inside another IF




day at a movie

theater. Everyone


free drink of their

choice, and kids

under 10 years old,

get a free gift. This is called a nested IF


Back To Grading



grade.

So, how do

this?





Back To Grading



grade.

We can use

a nested IF





Back To Grading

! Put code for other grades in the ELSE section of

our A? IF.


Back To Grading

! Put code for other grades in the ELSE section of

our A? IF.

give other grade

Code for all

other grades:

Either a B

or the rest

23


Back To Grading

! This is a nested IF.

give other grade


Back To Grading

! This is a nested IF. You can nest more than

once.

give other grade

Code to

give C or

other grade


Back To Grading

! This is a nested IF. You can nest more than

once.

give other grade

Nest it


Back To Grading

! This is an IF inside an IF inside an IF.

give other grade


Back To Grading

! One last time:

Give

D or Fgive other grade


Back To Grading

! One last time:

Give

D or Fgive other grade

24


Back To Grading

! One last time:


Back To Grading

! One last time:

! Note: This is

one big IF

statement{


Back To Grading

! One last time:

! Note: This isone big IFstatement

! Not fiveseparatestatements likebefore

{Jt Scratch Lesson 5 Fall 2011 • slide 142

Back To Grading

! One last time:

! Note: This isone big IFstatement

! Not fiveseparatestatements likebefore

{


Back To Grading

! Note: This is onebig IF statement

! As such, it isimpossible formore than onegrade to beassigned.


Back To Grading

! Note: This is one bigIF statement

! As such, it isimpossible for morethan one grade to beassigned.

! The one main IFeither Says A or

(else section) assignsone other grade

25


Back To Grading

! Note: This is onebig IF statement

! No ANDs needed.


Complete Code


Complete Code

Demo:

Grading 4


Main Point

! We already knew you use a regular IF if you

want to choose between doing something or

skipping it.


Main Point

! We already knew you use a regular IF if you

want to choose between doing something or

skipping it.

! And, if you need to choose between two things

to do, and you definitely want to do one of

those two things, you use an IF/ELSE


Main Point

! We already knew you use a regular IF if youwant to choose between doing something orskipping it.

! And, if you need to choose between two thingsto do, and you definitely want to do one ofthose two things, you use an IF/ELSE

! Now we know, if you have more than twochoices - a multiple choice decision - and youwant to do exactly one of the choices, then useNESTED IFs

26


Main Point

For multiple choice decisions, use NESTED IFs


One Last Fix

! Most people think of exactly 90 as an A, and

exactly an 80 as a B.

! What does our code do? Lets go see.


One Last Fix

! Most people think of exactly 90 as an A, and

exactly an 80 as a B.

! What does our code do? Lets go see.

! Some of you already noticed this.

! How do we fix this?


One Last Fix

! Most programming languages have more

relational operators, similar to what you

learned long ago in math class. Like:

IF Score >= 90


One Last Fix


relational operators, similar to what you learned

long ago in math class. Like: Score >= 90

! Scratch does not. So we have to use the logical

operators.


One Last Fix





operators.

! For example, assign an A when the grade is over

90 or when the grade is exactly 90.

27


One Last Fix





operators.



! If either one of those two possibilities is true, we

should give an A.


One Last Fix




should give an A.

! What logical (boolean) operator do you use when

just one of two possibilities has to be true?


One Last Fix




should give an A.

! What logical (boolean) operator do you use when

just one of two possibilities has to be true? OR


One Last Fix




should give an A.

! What logical (boolean) operator do you need just

one of two possibilities has to be true? OR


One Last Fix




Code to

Handle Exact

Scores

Notice ORs

28


Code to

Handle Exact

Scores

Notice ORs


Code to

Handle Exact

Scores

Notice ORs

Demo:

Grading 5


Adding Multimedia

! Stage


Adding Multimedia

! Stage

! Costumes


Adding Multimedia

! Stage

! Costumes

! Sounds


Adding Multimedia

! Change costume, play appropriate sound when

grade is assigned.

29


Code With

Multimedia

Included


Code With

Multimedia

Included

Demo:

Grading


Availability of Slides

Go to

nbcs.rutgers.edu/~jt

to see the powerpoint

slides and/or podcasts

for this lecture

Scratch Programming - Rutgers CS

Documents

Transcript of Scratch Programming - Rutgers CS